This invention relates to a process for the separation and recovery of strontium values from nuclear wastes. More specifically, this invention relates to an improved process for the separation and recovery of strontium values from nuclear waste reprocessing solutions containing these values together with actinide and other fission product values.
The removal of .sup.90 Sr from acidic high level liquid waste (HLLW) resulting from the processing of spent nuclear reactor fuels has long been recognized as a special problem. Strontium-90 and Cs.sup.137 are the two major generators of heat in nuclear waste. Thus, their presence complicates waste management options. The radioactive waste process stream from spent fuel reprocessing often contains the transuranium elements (TRUs) neptunium, plutonium and americium, in addition to strontium and other components, in a solution with a high (1-6M) nitric acid content. Because of the long-lived nature of the TRU elements, it is anticipated that they will have to be stored in geologically stable storage facilities for periods of up to one million years. The strontium in the waste stream, if left in the waste as it is solidified for storage, will generate a significant amount of thermal energy which increases the space required for the storage of the waste canister. Thus, it is imperative that a satisfactory method be found for the removal of the strontium from the high-level liquid wastes before they are solidified into a form suitable for long term storage.
Strontium-90, because of the heat which it generates, is also valuable as a reliable source of thermal energy for use in radioisotopic thermal electric generators.
U.S. Pat. No. 5,100,585, assigned to the U.S. Department of Energy, and incorporated herein by reference, described a process for the recovery of strontium and technetium values from acidic feed solutions containing other fission product values and containing up to 6 molar nitric acid. As taught therein, the strontium and technetium values are selectively extracted from the acidic feed solution using a macrocyclic polyether in a suitable diluent which may be an alcohol, ketone, carboxylic acid, or ester. From a practical standpoint however, the diluent of choice is 1-octanol.
However, none of the above diluents are compatible with the normal paraffinic hydrocarbon (NPH) diluents used in either the PUREX or the TRUEX processes, the primary processes utilized in the processing of nuclear reactor fuel. This lack of compatability can complicate attempts to develop processes in which both transuranic elements and radiostrontium are sequentially removed from a given waste stream. Then too, because of their superior physical properties (e.g., lower volatility and higher flash point) and lower cost, paraffinic hydrocarbon diluents are preferable in any large-scale process to the alcohols, ketones, carboxylic acids and esters suggested as diluents by the prior art.
We have found that by combining certain phase modifiers with a paraffinic hydrocarbon diluent, we are able to utilize the crown ethers of the prior art as an extractant for the selective recovery of strontium from waste nitric acid feed solutions containing strontium together with other fission product values.
BRIEF SUMMARY OF THE INVENTION
The invention therefore is an improved process for recovering strontium values from an aqueous nitric acid feed solution containing these and other fission product values by contacting the aqueous solution containing the values with an organic extractant solution consisting of a crown ether in a diluent, said diluent being selected from the group consisting of either a neat (i.e., undiluted) phase modifier or a paraffinic hydrocarbon having from 10 to 15 carbons containing a phase modifier, maintaining this contact for a period of time sufficient for the strontium values to be taken up by the extractant solution, and separating the loaded extractant solution from the aqueous solution, thereby separating the strontium values from the aqueous acid feed solution.
The process of the invention may be used with the non-transuranium element-containing high radioactive level nitric acid waste stream (the raffinate) from the TRUEX process or it may be used as a separate process to recover strontium values from nitric acid-containing waste streams.
It is therefore one object of the invention to provide a process for the recovery of strontium values from acidic waste solutions containing strontium values together with other fission product values.
It is another object of the invention to provide a process for the recovery of strontium values from aqueous nitric acid solutions.
It is still another object of the invention to provide a process for the recovery of strontium values from aqueous nitric acid solutions containing up to 6 molar nitric acid.
Finally, it is the object of the invention to provide a process which utilizes crown ethers for the recovery of strontium values from aqueous nitric acid waste streams containing up to 6 molar nitric acid.